Drilling jack-ups are in many cases fitted with a drilling derrick mounted on the aft end of a cantilever, which can be moved over the stern (or side) of the jack-up. In order to drill multiple wells or reach over an array of existing wells, the drilling tower derrick needs to cover a drilling envelope that is defined by a certain length and width.
The longitudinal movement of the drilling derrick is effectuated by moving the cantilever in the longitudinal direction. This is done by a skidding system, performing the so-called X-movement of the cantilever. In the floating condition of the jack-up, the cantilever is fully retracted. After elevation of the jack-up at the site, the cantilever can be moved in X-direction to the required longitudinal position.
The transverse movement (or Y-movement) of the drilling tower derrick can be created by shifting the drilling tower derrick in transverse direction over the cantilever, or by transverse shifting of the cantilever in which case the drilling derrick remains at its fixed position on the cantilever. The first method of transversely shifting the drilling derrick has severe drawbacks, especially the load reduction that is caused when the drilling derrick is shifted at a significant offset from its central position on the cantilever. Therefore the modern state-of-the-art cantilevers apply the transverse shifting of the whole cantilever.
The combined longitudinal and transverse cantilever movement can be effectuated as described in U.S. Pat. No. 6,171,027.
The transverse movement of the cantilever is limited by fixed structures on the maindeck, such as the jackhouses with the legs. This limit will be more restrictive in jack-ups which do not have a wide stern and consequently do not have a large spacing between the aft legs, such as is the case in ship shaped self-propelled jack-ups. As a consequence, the transverse movement of the drill tower will be limited in such jack-ups, and it may not be possible to reach the required width of the drilling envelope.
WO2004035985 describes a drilling rig comprising a cantilever which is movable in a first direction and a second direction, a supporting cart disposed between the cantilever and the drilling rig, wherein the first movement is a translational movement and the second movement is a rotational movement. Since the supporting cart is an additional structure that is connected to the drilling rig having a fixed point of rotation, it is a heavy structure reducing the load bearing capacity of the jack-up and/or the cantilever.
The present invention is intended to resolve at least one of the abovementioned drawbacks.